You might have ­assumed the ­decline of formal ­duelling, as a means of settling arguments, was a positive development. In former times, insulting someone's honour might lead to getting shot in the head, or sliced by a sword, whereas these days, except in the roughest neighbourhoods, neither's particularly likely. Yet as Bertrand Russell observed, duelling did serve a purpose: like the formal contests of wit in the court of Louis XVI, it permitted one to be "offensive without being ill-bred", delivering or replying to an insult without rupturing the social fabric. Duelling conformed to rules, and its decline, Russell wrote, made it "difficult to be insulting without being ungentle­manly", if also easier to be insulting without getting shot in the head.

It's an article of faith that we live in an era of unsurpassed rudeness – of shouting mobile phone users, inconsiderate drivers and shoulder-barging tube riders – but a moment's reflection confirms this rudeness is something very different from Russell's targeted insults. Instead, it's usually obliviousness, which is a trickier concept. Clearly, oblivious­ness is less malicious: the shouty mobile phone user isn't ­deliberately trying to annoy you. And a certain degree of obliviousness is vital to the harmonious functioning of crowded urban spaces. (Erving Goffman called it "civil inattention".) Yet, perversely, all this makes ­obliviousness-based rudeness more annoying. To be expertly, consciously insulted can be almost a pleasure, but to be the victim of obliviousness is not; at least the expert insulter does me the honour of acknowledging my existence. This must explain why I, and others, get riled by overenthusiastic "public displays of ­affection". If you carry on around me like you'd carry on on your sofa, you're effectively equating me with your sofa.

Social psychologists use the dull term "self-schema" to describe the mental maps we use to make sense of our own personalities: we think of ourselves as traditional or conven­tional, self-disciplined or lazy, optimistic or pessimistic. Yet when researchers try to measure such things, they run into people who are "aschematic": they've simply never given much thought to whether they're traditional or conventional, etcetera. So it is with oblivious rude­ness: your always-punctual friend may think of herself as conscientious, but your always-late friend has probably never considered the matter. The silver lining is that remedy­ing obliviousness, in principle, ought to be a simple matter of ­triggering a person's a­wareness. I'm not about to start picking fights with wearers of leaky headphones, but it's amazing how effective it can be to tap one's fingers amiably to the beat of the leaking music.

The vicious rudeness of much ­online discussion might seem like an exception to this rule, as it can feel very calculated and conscious. But something similar is surely involved: an obliviousness, albeit a partial one, to the fact that one's inter­locutors in cyberspace are real people. Not being a commentator on politics, I don't get many venomous emails from strangers, but when I do, it's usually clear the writer didn't think through the fact that a specific human would be reading their words. It is the purest of joys to respond to such emails – demurely, of course, yet by one's very demureness upbraiding the sender for rudeness. I like to think I'm being insulting without ­being ungentlemanly..